It is well known that generators have maximum and minimum real power capabilities. In addition, they also have maximum and minimum reactive power capabilities. The maximum reactive power capability
The Power Flow Problem
2
corresponds to the maximum reactive power that the generator may produce when operating with a lagging
power factor. The minimum reactive power capability corresponds to the maximum reactive power the
generator may absorb when operating with a leading power factor. These limitations are a function of the
real power output of the generator, that is, as the real power increases, the reactive power limitations move
closer to zero. The solid curve in Figure T7.1 is a typical generator capability curve, which shows the
lagging and leading reactive limitations (the ordinate) as real power is varied (the abscissa). Most power
flow programs model the generator reactive capabilities by assuming a somewhat conservative value for
Pmax (perhaps 95% of the actual value), and then fixing the reactive limits Qmax (for the lagging limit) and
Qming (for the leading limit) according to the dotted lines shown in Fig. T2.1.
Pmax